Recent work with probe molecules for the small pore system (hemepeptides) has led to the discovery of transendothelial channels formed by chains of fused plasmalemmal vesicles in the wall of muscle capillaries (rat diaphragm). Freeze cleaved preparations of reliably identified segments of the microvasculature (arterioles, capillaries, venules) have shown that there are characteristic segmental variations in the organization of cell junctions in the vascular endothelium. This and other findings suggest that the structural basis of permeability varies from segment to segment. This assumption was checked on the capillaries on postcapillary venules of the mouse diaphragm. In the former, macromolecular tracers are transported by plasmalemmal vesicles or move through transendothelial channels. In the latter, focally open junctions represent an additional passageway for molecules smaller than approximately 70A. The distribution of anionic sites on the luminal aspect of the plasmalemma was investigated with cationized ferritin as a tracer. The results revealed the existence of differentiated "microdomains" related to characteristic structural features of the endothelium. The highest concentration of acidic sites was found on fenestral apertures, where they were contributed primarily by sulfated glycosaminoglycans, (heparan sulfate or heparin). Anionic sites were not detectable on plasmalemmal vesicles, transendothelial channels and their stomatal apertures. It is proposed to continue the mapping of active sites on the surface of the endothelium isolate microvascular endothelia in sufficient amounts for cell fractionation and investigate modulations in vesicular transport induced by variations in blood pressure and body temperature.